This invention relates to phase transfer catalysis. More particularly, it relates to the preparation of aqueous solutions of hexasubstituted guanidinium salts useful as phase transfer catalysts, and to the use of such solutions in the preparation of polyetherimides and intermediates therefor.
The use of hexaalkylguanidinium salts as phase transfer catalysts in various reactions is known. Many of these reactions are characterized by the necessity of conducting them under anhydrous conditions. This makes it very difficult to introduce measured amounts of the hexasubstituted guanidinium salt into the reaction mixture, since such salts are solids and are only sparingly soluble in the organic solvents employed.
Hexasubstituted guanidinium salts may be prepared by a series of three reactions: the phosgenation of a secondary amine to produce a tetrasubstituted urea; further phosgenation of the tetrasubstituted urea to form a chloroformamidinium chloride, also known as a xe2x80x9cVilsmeier saltsxe2x80x9d and sometimes designated as such hereinafter; and finally, the reaction of the Vilsmeier salt with further secondary amine to produce the hexasubstituted guanidinium chloride. The third of these reactions must be conducted under anhydrous conditions, and the second is preferably anhydrous also.
It is frequently found that preparation of the Vilsmeier salt in good yield cannot be achieved unless expensive solvents such as acetonitrile are employed or a substantial excess of phosgene is used. This is true, for example, of the method described in Kantlehner et al., Liebigs Ann. Chem., 1984, 108-126, which afforded the hexasubstituted guanidinium salt in 94% yield but employed acetonitrile as the solvent. A method employing phosphorus oxychloride in toluene is described in Example 1 of U.S. Pat. No. 5,132,423; it afforded the product in 87% yield. A method employing phosgene and toluene, described in Barton et al., J. Chem. Soc., Perkin Trans., 1982, 2085-2090, resulted in a relatively low yield (85%) even with the use of a substantial excess of phosgene (molar ratio 1.95:1). The use of a large excess of phosgene, in any event, is undesirable since phosgene is toxic and is difficult to contain during the reaction. Moreover, disposal of a large excess of phosgene generally requires a sparging operation including passage through a caustic scrubber, presenting a potential hazardous waste problem upon disposal.
The product of the above-described series of reactions is invariably the hexasubstituted guanidinium chloride. It has further been found that the chlorides are hygroscopic, making their use under anhydrous conditions complicated. Therefore, they are often converted to other salts such as the bromides. This necessitates an additional reaction step, which is also expensive and therefore undesirable.
The use of hexasubstituted guanidinium salts as phase transfer catalysts in the reaction of bisphenol salts with halo- or nitro-substituted phthalimides to form bisimides, which are conventional intermediates for polyetherimide production, is described in Example 4 of U.S. Pat. No. 5,081,298. Its use in the direct preparation of polyetherimides by the reaction of bisphenol salts with similarly substituted bis(phthalimido) compounds is disclosed in U.S. Pat. No. 5,229,482. In each case, the guanidinium salts were added in solid form, which, as previously mentioned, presents numerous problems in commercial operations. Anhydrous conditions are mandatory in these reactions, since the presence of even traces of water can drastically decrease yields.
Methods of preparing hexasubstituted guanidinium salts in high yield and using them in a convenient manner as phase transfer catalysts in commercial operations, therefore, continue to be desirable subjects of investigation.
The present invention has numerous aspects relating to the preparation of hexasubstituted guanidinium salts and intermediates therefor, compositions comprising such salts and the use of such compositions as phase transfer catalysts in the preparation of polyetherimides and intermediates therefor. Each aspect is summarized immediately below.
First aspect: a method for preparing an aqueous solution of a hexasubstituted guanidinium salt which comprises the steps of:
(A) passing phosgene into a mixture comprising an aliphatic or alicyclic secondary amine, water and an organic solvent of low polarity at a reaction temperature in the range of about 40-80xc2x0 C. and a pH in the range of about 10-13 maintained by the addition of an alkali metal base; the proportion of water being effective to produce a solution of by-product alkali metal chloride having a concentration of at least about 95% by weight of the saturation concentration under the prevailing reaction conditions, the ratio of equivalents of phosgene to said amine being in the range of 1.0-1.15:1, the volume ratio of organic solvent to water being in the range of about 0.8-5.0:1 and the proportion of amine being calculated to afford about a 25-90% by weight solution of product in said organic solvent assuming complete conversion, to produce a first product mixture comprising tetrasubstituted urea;
(B) removing water, amine and alkali metal chloride from said first product mixture to produce a first product mixture;
(C) adjusting said first product mixture to a tetrasubstituted urea concentration in the range of about 40-80% by weight by the addition or removal of organic solvent if necessary;
(D) introducing phosgene at a temperature in the range of about 60-80xc2x0 C., the proportion of phosgene added being effective to react with any water present plus a ratio of equivalents of phosgene to said tetrasubstituted urea in the range of 1.05-1.15:1, to produce an anhydrous mixture comprising tetrasubstituted chloroformamidinium chloride;
(E) removing any unreacted phosgene from said anhydrous mixture;
(F) adding secondary amine to said anhydrous mixture in an amount effective to convert said tetrasubstituted chloroformamidinium chloride to hexasubstituted guanidinium salt; and
(G) diluting said hexasubstituted guanidinium salt with water in a basic medium to produce aqueous and organic phases, removing said organic phase comprising unreacted secondary amine, and acidifying said aqueous phase containing said hexasubstituted guanidinium salt as necessary to adjust its pH to a value in the range of 2-12.
Second and third aspects: methods for preparing a tetrasubstituted urea which comprises step A, and for preparing a tetrasubstituted chloroformamidinium chloride which comprises steps D and E, all as defined above.
Fourth aspect: a method for preparing an aqueous solution of a hexasubstituted guanidinium salt having one or more chains containing at least 3 carbon atoms attached to each nitrogen atom which comprises the steps of:
(H) passing phosgene into a mixture comprising an aliphatic or alicyclic secondary amine, water and an organic solvent of low polarity at a reaction temperature in the range of about 40-80xc2x0 C. and a pH in the range of about 10-13 maintained by the addition of an alkali metal base, to produce a first product mixture comprising tetrasubstituted urea;
(J) removing water, amine and alkali metal chloride from said first product mixture to produce a first product mixture;
(K) introducing phosgene at a temperature in the range of about 60xe2x80x9480xc2x0 C. to produce an anhydrous mixture comprising tetrasubstituted chloroformamidinium chloride;
(L) removing any unreacted phosgene from said anhydrous mixture;
(M) adding secondary amine to said anhydrous mixture in an amount effective to convert said tetrasubstituted chloroformamidinium chloride to hexasubstituted guanidinium salt in a second product mixture;
(N) diluting said second product mixture with an amount of water in a basic medium, said amount of water being effective to cause the formation of three liquid phases, isolating the middle phase comprising said hexasubstituted guanidinium salt and acidifying said middle phase as necessary to adjust its pH to a value in the range of 2-12.
Fifth aspect: a composition comprising an aqueous solution of a hexasubstituted guanidinium chloride having a pH in the range of 2-12.
Sixth aspect: a composition comprising a hexasubstituted guanidinium chloride in aqueous solution and at least one of the following:
an imide having the formula 
wherein A is a trivalent aromatic radical, R1 is an unsubstituted or substituted mono- or divalent hydrocarbon radical containing about 1-13 carbon atoms, X1 is halo or nitro and q is 1 or 2; and
an alkali metal salt of a dihydroxyaromatic compound.
Seventh aspect: a method for preparing a polyetherimide or intermediate therefor which comprises the steps of:
(P) removing substantially all water from a mixture comprising an aqueous solution of a hexasubstituted guanidinium chloride to form an anhydrous mixture comprising said hexasubstituted guanidinium chloride;
(R) preparing, in an organic solvent of low polarity, an anhydrous mixture comprising said hexasubstituted guanidinium chloride; an imide having formula I and an alkali metal salt of a dihydroxyaromatic compound; and
(S) effecting reaction in said anhydrous mixture between said alkali metal salt and said imide.
The hexasubstituted guanidinium (hereinafter sometimes simply xe2x80x9cguanidiniumxe2x80x9d for brevity) chlorides which are the subject of the present invention include all those disclosed in the aforementioned U.S. Pat. No. 5,132,423, the disclosure of which is incorporated by reference herein. In preferred embodiments, the guanidinium chlorides have the formula
[(R2)2N]3C⊕Cxe2x8ax96,xe2x80x83xe2x80x83(II)
wherein R2 is a C1-6 primary alkyl radical or two R2 values with the connecting nitrogen atom form a saturated heterocyclic radical. Such compounds may be prepared by the above-described three-step method wherein the secondary amine has the formula (R2)2NH.
The first aspect of the invention is chiefly effective for production of compositions comprising hexaethylguanidinium chloride, by reason of its solubility characteristics; however, the invention also contemplates its use for other guanidinium chlorides. In step A, phosgene is passed into a mixture of amine, water and an organic solvent of low polarity. Suitable organic solvents include aliphatic and aromatic hydrocarbons and halogenated aromatic hydrocarbons, as illustrated by benzene, toluene, xylene, chlorobenzene, o-dichlorobenzene, chlorotoluene, dichlorotoluene and octane. Aromatic solvents which form azeotropic mixtures with water are preferred; these include benzene, toluene and xylene. Thus, aromatic hydrocarbons are particularly preferred and toluene is most preferred.
The conditions of the reaction between phosgene and the secondary amine are critical for obtaining a high yield of tetrasubstituted urea. These conditions include a pH within the range of about 10-13, preferably about 10.5-12.5, maintained by the addition of an alkali metal base, typically sodium hydroxide or potassium hydroxide and especially the former; a reaction temperature of about 40-80xc2x0 C., preferably about 45-55xc2x0 C.; and a ratio of equivalents of phosgene to amine in the range of 1.0-1.15:1, preferably about 1.1:1. As used herein, the word xe2x80x9cequivalentxe2x80x9d designates a number of parts by weight of said compound equal to its molecular weight divided by the number of reactive groups therein under the prevailing reaction conditions. Thus, one equivalent of a secondary amine is equal to its molecular weight and one equivalent of phosgene is half its molecular weight.
Another crucial condition is the proportion of amine with respect to organic solvent, which should be calculated to afford about a 25-90% (by weight), preferably 35-85%, solution of tetrasubstituted urea in the organic solvent, assuming complete conversion has taken place. Suitable proportions of amine can be readily determined by calculation or simple experimentation.
Still another important condition is the volume ratio of organic solvent to water, which should be in the range of about 0.8-5.0:1 and especially about 1.5-4.5:1. Under most circumstances, said ratio will relate inversely to the concentration of calculated product in organic solvent. Thus, at calculated concentrations of 30% and 60% typical volume ratios are 4:1 and 2:1, respectively.
Finally, it is required that the proportion of water be low enough that by-product alkali metal chloride, usually sodium chloride, be present in an amount equal to at least about 95% of the saturation amount for the aqueous system under the prevailing reaction conditions, including temperature. It is practical, and in certain instances perhaps preferred, to maintain saturation or even supersaturation conditions so that at least a portion of the alkali metal chloride generated precipitates out and essentially all amine and product tetrasubstituted urea remain in the organic phase. This simplifies the problem of disposal of waste products, which typically include the aqueous phase when the reaction is complete.
The reaction between the amine and phosgene takes place in two steps, the first being the formation of a substituted carbamoyl chloride and the second the reaction of said carbamoyl chloride with further amine to form tetrasubstituted urea. The reaction is deemed complete when no organically bound chlorine remains in the mixture or the value of organically bound chlorine has reached a minimum. If the described conditions are maintained, yields are typically above 90%.
Following completion of the reaction, it is usually preferred to add water to dissolve any alkali metal chloride which may have precipitated. Then, in step B, water, amine and alkali metal chloride are removed from the first product mixture formed in step A. Since the second reaction forming the Vilsmeier salt proceeds most efficiently under anhydrous conditions, it is preferred to remove substantially all water. Typically, the water content of the anhydrous mixture will be no greater than 50 ppm (by weight).
The major proportion of water and alkali metal chloride may be removed by merely allowing the aqueous and organic phases to separate and removing the aqueous layer. Final traces of water are preferably removed by azeotropic distillation, which is the reason the preferred organic solvents are those which form azeotropic mixtures with water. Any remaining traces of amine will also be removed by the distillation. Often, solid alkali metal chloride will be present in addition to the material dissolved in the aqueous phase, and it may be removed by filtration following the distillation operation.
In step C, the concentration of tetrasubstituted urea in the anhydrous mixture is adjusted to about 40-80% by weight by adding organic solvent or by removal of solvent, typically by distillation. This step is, of course, unnecessary if the tetrasubstituted urea concentration is already within the mandated range.
In step D, phosgene is introduced into the anhydrous mixture at a temperature in the range of about 60-80xc2x0 C. The ratio of equivalents of phosgene to tetrasubstituted urea when phosgene addition is complete is in the range of about 1.05-1.15:1. The progress of the reaction may be monitored by conventional analytical techniques such as nuclear magnetic resonance spectroscopy, with the mixture being maintained within the 60-80xc2x0 C. temperature range until the reaction is complete. At that point, unreacted phosgene is removed in step E. xe2x80x9cRemovalxe2x80x9d as defined herein includes removal by reaction, in this step typically by introduction of a quenching proportion of amine, and volatilization as by sparging with nitrogen.
The final reaction, between the Vilsmeier salt and further amine, is conducted in step F. The temperature of this step is typically in the range of about 45-70xc2x0 C.
Under the preferred conditions where the reaction mixture is anhydrous, the ratio of equivalents of amine to Vilsmeier salt is in excess of 2:1. When the reaction mixture is anhydrous, said ratio is most often in the range of about 2.05-2.5:1. One equivalent of the amine reacts with the Vilsmeier salt to form hexasubstituted guanidine, and another equivalent neutralizes the hydrogen chloride thus generated. If the mixture is not anhydrous, additional phosgene will be required to react with and thus remove any water present.
A convenient means for determining when the Vilsmeier salt has been totally converted to hexasubstituted guanidine is to employ phosgene paper. Unreacted Vilsmeier salt produces a red color upon contact therewith. Thus, the reaction is complete when no such color is produced.
Step G permits isolation of the guanidinium chloride in aqueous solution, said solution also containing alkali metal chloride which is typically present in the amount of about 8-12% by weight, based on total solution. Isolation is achieved by adding an appropriate volume of water in a basic medium, typically at a pH of at least about 13, and removing excess amine, typically by distillation of primarily the organic phase. Following distillation, further water may be added if necessary. The desired guanidinium chloride, being water-soluble, remains in the aqueous phase and the organic phase may be removed and discarded. To maintain the acid-base balance of the resulting aqueous product during employment as a phase transfer catalyst, acid, preferably dilute aqueous mineral acid and typically hydrochloric acid, is finally added to bring the pH of the solution to a value in the range of 2-12, preferably 6-8.
In addition to optimization of the yield of guanidinium chloride, a further advantage of the procedure described hereinabove is that unreacted amine is removed at various stages by distillation. Thus, it may be recovered and recycled.
The second and third aspects of the invention are based on the possible need under certain conditions to stop the process after the preparation of tetrasubstituted urea and/or Vilsmeier salt and store the products. Thus, step A may be isolated and employed separately, as may the combination of steps D and E.
The fourth aspect of the invention is based on three further discoveries. First, guanidinium chlorides having at least three carbon atoms in the chain(s) (whether acyclic or alicyclic) attached to each nitrogen atom (hereinafter sometimes xe2x80x9chigher guanidinium chloridesxe2x80x9d) frequently have reactivity and solubility properties substantially different from those of hexaethylguanidinium chloride. Thus, it is frequently found that the conditions hereinabove described in detail are not necessary or optimal for their preparation. This is particularly true of the concentrations and proportions in steps A-D.
Second, although in the use of the product as a phase transfer catalyst the presence of alkali metal chloride is frequently innocuous, there are circumstances under which it may be detrimental. For example, its presence may require further dilution with water to maintain the solution homogeneous, thus complicating such operations as packaging, handling, shipping and storage. Also, aqueous alkali metal chlorides, especially sodium chloride, are corrosive to equipment made of certain materials such as stainless steel and their presence may therefore be undesirable if such equipment is to be employed. Finally, the presence of alkali metal chlorides in the product formed in the phase transfer catalysis reaction may cause by-products such as sodium nitrite, otherwise marketable for other purposes, to have an excessive chloride ion concentration which prevents them from being saleable. Under circumstances such as these, it may be advantageous to employ a higher guanidinium chloride which is readily separated from alkali metal chloride.
Third, if the proportions of water and organic solvent are suitably adjusted during the concluding operations of the method for preparing higher guanidinium chlorides, the mixture containing the product separates into three phases. The top phase comprises organic solvent and by-products soluble therein. The bottom phase consists essentially of alkali metal chloride brine. The middle phase is essentially free from alkali metal chloride and contains the desired guanidinium chloride, as well as a minor proportion of water.
These findings call forth the fourth aspect, which is a method for preparing higher guanidinium chlorides having steps similar to those of the first aspect but without some of the compositional limitations. It also includes the step of diluting the mixture after formation of the guanidinium salt to cause formation of the aforementioned three phases and separating the middle phase, which comprises said guanidinium salt and water and is substantially free from alkali metal chloride.
The fifth aspect of the invention is directed to the aqueous guanidinium salt solutions obtained according to the first and fourth aspects. The pH of said solutions has been adjusted as previously described.
The sixth aspect is based on the mode of use of the aqueous guanidinium chloride solutions as phase transfer catalysts and its nature is evident from the description hereinafter of the seventh aspect.
That aspect is the use of aqueous solutions of guanidinium chlorides as source materials for the phase transfer catalysts employed in displacement reactions for the formation of polyetherimides and intermediates therefor. As previously mentioned, employment of these guanidinium chlorides in solid form is difficult because they are hygroscopic. However, their conversion to less hygroscopic salts such as bromides requires an additional process step and is thus undesirable for commercial purposes. Thus, it is very advantageous to be able to introduce them as aqueous solutions and subsequently remove water.
In the imide represented by formula I, the A radical generally contains about 6-30 carbon atoms. The imide is generally derived from an o-dicarboxylic acid such as phthalic acid or 2,3-naphthalenedicarboxylic acid; however, derivatives of acids such as 1,8-naphthalenedicarboxylic acid are also suitable. Most preferably, the imide is a phthalimide.
When q is 1, the R1 value is preferably an alkyl and especially a lower alkyl radical (i.e., one containing up to 7 carbon atoms). Most preferably, it is methyl, n-propyl or n-butyl. When q is 2, it is preferably an arylene and especially a phenylene radical, most often m-phenylene, p-phenylene or a mixture thereof.
The bisphenol salts are generally compounds of the formula
R2(ZM)2,xe2x80x83xe2x80x83(III)
wherein R2 is a divalent aromatic radical containing about 6-30 carbon atoms, M is an alkali metal and Z is oxygen or sulfur. The R2 radical may be a hydrocarbon radical or may contain other atoms such as oxygen or sulfur. Illustrative dihydroxyaromatic compounds from which the R2 radicals may be derived are resorcinol, hydroquinone, 4,4xe2x80x2-dihydroxybiphenyl, 4,4xe2x80x2-dihydroxy-3,3xe2x80x2,5,5xe2x80x2-tetramethylbiphenyl, bis(4-hydroxyphenyl)methane, 3-hydroxyphenyl-4-hydroxyphenylmethane, 2,2-bis(2-hydroxyphenyl)propane, 2,2-bis(4-hydroxyphenyl)propane (hereinafter xe2x80x9cbisphenol Axe2x80x9d), 2-(3-hydroxyphenyl)-2-(4-hydroxyphenyl)propane, 1,1-bis(4-hydroxyphenyl)ethane, 2,2-bis(4-hydroxyphenyl)pentane, 1,1-bis(4-hydroxyphenyl)ethylene, 4,4xe2x80x2-dihydroxybenzophenone, bis(4-hydroxyphenyl)ether, bis(4-hydroxyphenyl) sulfide, bis(4-hydroxyphenyl) sulfoxide, bis(4-hydroxyphenyl) sulfone and 3-hydroxyphenyl-4-hydroxyphenyl sulfone.
The preferred R2 radicals are usually 
wherein each R3 is independently hydrogen or methyl, R4 is a straight-chain or branched alkylene radical containing 1-5 carbon atoms and each X2 is independently hydrogen or halogen (usually chlorine or bromine). Mixtures of the foregoing formulas are also contemplated. Especially desirable are the bisphenol A salts, having formula VI in which R4 is isopropylidene and each X2 is hydrogen.
The alkali metal in the bisphenol salt may be any of the known alkali metals. Sodium and potassium are usually preferred by reason of availability and low cost, with sodium being especially preferred. The Z value may be oxygen or sulfur and is usually oxygen.
The seventh aspect is applicable in at least two contexts. The first is the displacement reaction of halo- or nitro-substituted phthalimides with alkali metal salts of dihydroxyaromatic compounds (hereinafter sometimes simply xe2x80x9cbisphenol saltsxe2x80x9d) in organic solvents of low polarity such as those previously enumerated to produce bis(phthalimido)aromatic compounds; these may be further converted to dianhydrides which undergo reaction with diamines to form polyetherimides. The second is the direct reaction, in similar solvents, of bis(N-substituted phthalimido) compounds having similar substituents, as illustrated by 1,3-bis[(N-(4-chlorophthalimido)]benzene also known as 2,2xe2x80x2-(1,3-phenylene)bis[5-chloro-1H-isoindole-1,3(2H-dione)], with bisphenol salts for the direct formation of the polyetherimides.
It is known that the presence of water in any measurable proportions substantially decreases yield in these reactions. In general, the water content should be below 50 ppm by weight and it is strongly preferred that the reaction mixtures be maintained totally anhydrous. This is most often achieved by combining any reagent which may contain traces of water with an organic solvent which forms an azeotrope with water, as exemplified by the solvents of this type enumerated hereinabove, and removing the water by azeotropic distillation before introducing the other reagent.
The invention permits the guanidinium chloride to be introduced in aqueous solution followed by removal of water from the mixture containing it, typically by azeotropic distillation. In one embodiment, the aqueous solution, with alkali metal chloride present, is azeotropically distilled with toluene or the like prior to introduction of imide and bisphenol salt. In a related embodiment, and provided enough hot toluene is employed to dissolve the guanidinium chloride after distillation of the water (i.e., about 250 ml/g), the toluene solution may be filtered while hot to remove the alkali metal chloride.
For the most part, however, there is no advantage in segregating the phase transfer catalyst from both reagents before removal of the water since a principal advantage of the invention is to introduce it in liquid form. Thus, the often preferred embodiment is the one which also gives rise to the sixth embodiment: water removal is performed on a mixture of phase transfer catalyst with imide and/or bisphenol salt in combination with a solvent such as toluene. When substantially all water has been removed (xe2x80x9csubstantiallyxe2x80x9d meaning a total water content not greater than 50 ppm by weight), the other reagent may be added and the displacement reaction conducted under conventional conditions. If removal of water can be completed rapidly, the guanidinium chloride may be combined with both reagents. In these embodiments, step P is combined with at least a portion of step R.
In step S, the reaction mixture is normally heated at a temperature in the range of about 100-200xc2x0 C., preferably about 125-175xc2x0 C., for product formation. It is preferred to use stoichiometric amounts of the bisphenol salt and imide, but under appropriate conditions an excess of one reagent or the other, generally not more than about 25%, may be employed. An internal standard may be incorporated in the reaction mixture for analytical purposes. The proportion of guanidinium salt is a catalytically effective proportion, most often about 0.1-5.0 mole percent based on imide.
Isolation of the product may be achieved by conventional methods. These typically involve washing with an aqueous alkaline solution followed by drying of the organic phase and solvent stripping.
The invention is illustrated by the following examples.